Why can it take days to notice that a vehicle has been filled with the wrong fuel?

With reference to DS Wellesbourne wondering why it took five days for misfuelling a diesel to manifest itself, it would be because of the petrol being much lighter (typical density 07300) than the diesel (typical density 08400) already in his tank. The petrol would simply float on top of the diesel until the diesel in the tank had been used before the engine started burning the 37 litres of petrol. Without mechanical mixing (i.e. something shaped like a ship's propeller) it is very difficult to get to oil compounds with such a large difference in density to homogenise.

Asked on 17 March 2012 by GW, via email

Answered by Honest John
I wondered about that when I misfuelled in the summer. So what I did was added Wynns lubricity enhancer and continually refilled with V-Power diesel for 2,000 miles so I never got down to raw petrol. Got away with 9.3 litres of petrol in a 60-litre tank. Of course, in a moving car, the fuel gets shaken about. So only if the car is driven fairly sedately will the petrol remain on top in motion. I could not siphon the petrol from the top of the diesel because the tank is anti-siphon.
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